The present invention relates to a low-cost lithium ion secondary battery with high safety during overcharge, to a separator employed therein, to a battery pack and to electric/electronic devices provided with the lithium ion secondary battery, and to a charging method for the lithium ion secondary battery.
With the recent popularity and increasing performance of portable electronic devices there has been a commensurate demand for secondary batteries with high energy density. This demand has been met by greater use of lithium ion secondary batteries which employ a carbon material that can be electrochemically doped and dedoped with lithium as the negative electrode active material and a lithium-containing transition metal oxide as the positive electrode active material.
This type of lithium ion secondary battery undergoes charge and discharge by migration of lithium ions between the positive and negative electrodes, by which storage and release of electrical energy are accomplished. The lithium ion secondary battery has a high energy density, because it outputs an average voltage of approximately 3.7 V which is about 3 times that of conventional secondary batteries, but, because aqueous electrolyte solutions cannot be used as for conventional secondary batteries, non-aqueous electrolyte solutions with sufficient oxidation-reduction resistance are used. For this reason, lithium ion secondary batteries are often referred to as non-aqueous secondary batteries.
Because non-aqueous secondary batteries use a flammable non-aqueous electrolyte solution as the electrolyte solution, there is a risk of combustion and, therefore, great caution must be taken for safety when they are used. While numerous situations can result in exposure to risk of combustion, overcharging is particularly dangerous.
In order to prevent overcharging, existing non-aqueous secondary batteries are charged with a constant current and constant voltage, and the batteries are provided with a precise protection circuit (safety circuit: IC+FETxc3x972). Such protection circuits are costly and thus add to the cost of non-aqueous secondary batteries.
When overcharging is prevented by a protection circuit, the protection circuit sometimes fails to operate properly, and therefore it cannot be considered substantially safe. Existing non-aqueous secondary batteries include modifications such as the provision of a safety vent, PTC element and the use of a separator with a heat fuse function (shut down function) in order to safely destroy the battery in the event of overcharging if the protection circuit breaks during overcharging. However, even when such a means is provided, the safety during overcharging cannot always be reliably guaranteed, depending on the overcharging conditions and, in fact, combustion accidents still occur with non-aqueous secondary batteries.
Since safety measures against overcharging with non-aqueous secondary batteries are therefore still inadequate from the standpoint of safety and cost, a problem to be solved has remained, and various methods have been proposed to improve the problem.
One approach for improvement is aimed at destroying the battery in a safer manner when the protection circuit fails to function. Examples of this approach include addition of a compound that readily generates a gas upon overcharging and causes rapid actuation of a safety vent, as proposed in Japanese Patent No. 2928779, Japanese Patent No. 3061759, Japanese Patent No. 3113652, Japanese Unexamined Patent Publication No. 2000-306610 and elsewhere, addition of a compound that polymerizes upon overcharging, thus blocking the current, as proposed in Japanese Patent No. 3061756, and addition of a compound with an endothermic effect in the event of overcharging, as proposed in Japanese Unexamined Patent Publication No. 11-45740; some of these methods involving additives have been implemented and have improved the safety of non-aqueous secondary batteries.
Another approach is aimed at ensuring safety while also achieving cost savings by removal of the protection circuit or simple-protection as with a thermistor system. Examples of this approach include the use of redox shuttle additives, as proposed in Japanese Unexamined Patent Publication No. 6-338347, Japanese Unexamined Patent Publication No. 2000-251932, Japanese Unexamined Patent Publication No. 2000-277147, Japanese Unexamined Patent Publication No. 2000-228215 and elsewhere. Redox shuttle additives cause an oxidation-reduction reaction between the positive and negative electrodes upon overcharging, thus preventing overcharging by a mechanism which consumes the overcharge current. Some such additives have been implemented and have contributed to the improved safety of non-aqueous secondary batteries, but they have not succeeded in removing the protection circuit or simplifying it.
Japanese Unexamined Patent Application No. 2000-67917 proposes a technique related to preventing overcharging by employing a gel-polymer electrolyte film, and this suggests the possibility of eliminating or simplifying the protection circuit. However, the technique requires a film thickness of no less than 30 xcexcm for the gel-polymer electrolyte film, with 40 xcexcm or greater needed to obtain an adequate effect and even greater thicknesses in order to achieve a notable effect. This thickness is not widely used, though, considering that the separator thickness in most existing non-aqueous secondary batteries is 25 xcexcm, and that the trend is toward a smaller separator thickness as increasingly higher energy density is pursued for batteries.
Japanese Unexamined Patent Application No. 2000-123824 also proposes a technique for preventing overcharging that employs a gel-polymer electrolyte, thus suggesting the possibility of eliminating or simplifying the protection circuit. According to this technique, overcharging is prevented by using a polyether oligomer, but the technique is not widely used because of the resulting very poor discharge characteristics of the battery compared to existing non-aqueous secondary batteries.
It is an object of the present invention to solve the above-mentioned problems of the prior art by providing a non-aqueous secondary battery which maintains practical battery characteristics while permitting elimination of the protection circuit or simple-protection thereof to a heat-sensitive switch system such as a thermistor and/or PTC element, thus increasing safety during overcharging and lowering cost compared to conventional non-aqueous secondary batteries.
In order to solve these problems, the invention provides a lithium ion secondary battery comprising a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, wherein
1) the separator is composed essentially of a porous sheet,
2) the positive electrode active material and the negative electrode active material can be reversibly doped and dedoped such that, where Qp (mAh) is an electric charge necessary for causing total lithium contained in the positive electrode to be dedoped and Qn (mAh) is an electric charge necessary for causing lithium to fully dope the negative electrode, Qp greater than Qn, and
3) when the battery is charged at a charging current Ic (mA) in a range of 0.2 Qn/h less than Ic less than 2 Qn/h, in a range of an electric charge for charging Qc (mAh) of 1 less than Qc/Qn less than Qp/Qn, doping of the positive electrode by lithium is started through lithium particles produced on the negative electrode by charging of the battery and is continued up to Qc greater than Qp.
The invention further provides a lithium ion secondary battery pack comprising the aforementioned lithium ion secondary battery and a heat-sensitive switch system such as a thermistor and/or PTC element.
The invention still further provides a method for charging the aforementioned lithium ion secondary battery, which method comprises charging the lithium ion secondary battery by a constant current method, and determining completion of charging based on at least one of the following: battery temperature increase, battery voltage drop or battery voltage oscillation.
The invention still further provides electric/electronic devices comprising the aforementioned lithium ion secondary battery or lithium ion secondary battery pack.
The invention still further provides a lithium ion secondary battery separator comprising a sheet (A) with an average film thickness of 10-35 xcexcm, a basis weight of 6-20 g/m2, a gas permeability (JIS P8117) of no longer than 100 seconds, a MacMullin number of 10 or smaller and a MacMullin numberxc3x97average film thickness value of no greater than 200 xcexcm.
The invention still further provides a lithium ion secondary battery separator comprising a porous film with an average film thickness of 10-35 xcexcm and a basis weight of 10-25 g/m2, which contains a porous organic polymer film (B) that surrounds the above-mentioned sheet (A) and is swellable with the electrolyte solution and is retainable.